British police on Thursday opened a criminal investigation into a top Brexit donor over his alleged use of offshore companies to finance the campaign to leave the European Union.

The probe came after the Electoral Commission said it had "reasonable grounds" to believe Arron Banks -- an insurance company owner who was the single biggest donor behind the campaign to leave the EU -- was not the true source of the loans made to the pro-Brexit camp.

The National Crime Agency (NCA) said its "investigation relates to suspected electoral law offences... as well as any associated offences" by Banks and two companies he is associated with -- Better for the Country (BFTC) and campaign group Leave.EU.

They will also investigate Leave.EU chief Elizabeth Bilney.

The NCA is in charge of investigating serious and organised crime.

The probe into Arron Banks comes in the wake of media revelations that he held a series of secret meetings with senior Russian embassy officials in London around the time of the 2016 vote.

Banks denied any criminal wrongdoing following the reports.

At least one of those meetings allegedly saw the embassy officials present Banks with a lucrative investment opportunity in a Russian gold company, according to media reports.

The deal never went through and Russia denies backing the pro-Brexit effort in a bid to weaken the European Union.

Banks said on Thursday that he was happy to have the opportunity to clear his name.

"I am confident that a full and frank investigation will finally put an end to the ludicrous allegations levelled against me and my colleagues," Banks said in a statement issued by Leave.EU.

"I am a UK taxpayer and I have never received any foreign donations," he said.

Banks later sent a series of tweets that included a selfie taken in what he said was Bermuda.

"Gone fishing!" one of the tweets said.

Shell companies

The Electoral Commission said its investigation into alleged campaign violations focused on £2 million (2.3 million euros, $2.6 million) reported to have been loaned to BFTC by Banks and his insurance companies.

They also looked into a further £6 million reported to have been given to the organisation on behalf of Leave.EU by Banks alone.

The Electoral Commission said it had "reasonable grounds to suspect that Mr Banks was not the true source of the £8m loans made to Better for the Country".

It further believes that the loans involved companies incorporated in the British overseas territory of Gibraltar and the Isle of Man -- a crown dependency -- which is not permissible under British electoral law.

"Arron Banks, Elizabeth Bilney and others involved in Better for the Country, Leave.EU and associated companies concealed the true details of these financial transactions," the Electoral Commission said.

The commission's legal counsel Bob Posner said at least £2.9 million of the money funnelled into Leave.EU "was used to fund referendum spending and donations during the regulated period of the EU referendum".

"The financial transactions we have investigated include companies incorporated in Gibraltar and the Isle of Man," Posner said.

Banks said in his statement that the Electoral Commission was acting "under intense political pressure from anti-Brexit supporters".

He also argued that it was unfair that US billionaire George Soros was not targeted for his involvement in the campaign to stay in the European Union.

The Guardian newspaper noted that the Hungarian-American financier made his donations outside the official election period that bans foreign contributions.